Mouse mammary tumor virus (MMTV) is expressed in splenic B cells of several mouse strains, as evidenced by immunological means as a cell surface membrane antigen. We plan to study this antigen(s) in terms of its relation to virion polypeptides, the RNA encoding for its translation and importantly whether endogenous or exogenous MMTV genomic sequences encode for its expression. These parameters will be evaluated in low and high incidence breast cancer strains throughout the lifespan of the animals. We hypothesize that MMTV infection of splenic B-cells may be an important factor in the immunologic responsiveness of the host to viral-associated tumor antigens and to the natural immunity of hosts. Using various preparations of the cell membrane antigen we will evaluate immunologic responsiveness using both humoral and cell-mediated immunoassays. If evidence of a significant role in tumor immunity is found, the cell membrane antigen or cells will be used to manipulate host responses to relevant viral-associated breast tumor antigens.